Racetrack memory is a memory storage device that stores data in the form of magnetic domain walls that separate magnetic regions magnetized in oppositely oriented directions (for example, see U.S. Pat. Nos. 6,834,005, 6,920,062, 7,551,469 and 8,638,601 to Parkin and co-workers, which are hereby incorporated by reference). As described in these patents, FIG. 1 (FIGS. 1A and 1B) illustrates an exemplary high-level architecture of a magnetic memory system 100 comprising a magnetic shift register 10 that utilizes a writing device (also referred to herein as writing element) 15 and a reading device (also referred to herein as reading element) 20. Both the reading device 20 and the writing device 15 form a read/write element of system 100.
The magnetic shift register 10 comprises a fine track 11 made of ferromagnetic material. The track 11 can be magnetized in small sections, or domains, in one direction or another. Information is stored in regions such as domains 25, 30 in the track 11. The order parameter of the magnetic material from which the track is fabricated, that is, the magnetization direction or the direction of the magnetic moment, changes from one direction to another. This variation in the direction of the magnetic moment forms the basis for storing information in the track 11. In one embodiment, the magnetic shift register 10 comprises a data region 35 and a reservoir 40. The data region 35 comprises a contiguous set of domains such as domains 25, 30 that store data. Additional length is provided to the magnetic shift register 10 in the form of a reservoir 40.
The reservoir 40 is made sufficiently long so that it accommodates all the domains in the region 35 when these domains are moved completely from region 35 across the writing and reading elements for the purposes of writing and reading domains into region 40. At any given time, the domains are thus stored partially in region 35 and partially in region 40, so it is the combination of region 35 and region 40 that forms the storage element. In one embodiment, the reservoir 40 is devoid of magnetic domains in a quiescent state.
Thus, the storage region 35 at any given time may be located within a different portion of the magnetic shift register 10 and the reservoir 40 would be divided into two regions on either side of the storage region 35. Although the storage region 35 is one contiguous region, and in one embodiment of this application the spatial distribution and extent of the domains within the storage region 35 would be approximately the same no matter where the storage region 35 resides within the shift register 10, in another embodiment portions of the storage region may be expanded during the motion of this region particularly across the reading and writing elements. A portion or all of the data region 35 is moved into the reservoir 40 to access data in specific domains.
The reservoir 40 is shown in FIG. 1A as being approximately the same size as the data region 35. However, other alternative embodiments may allow the reservoir 40 to have a different size than the data region 35. As an example, the reservoir 40 could be much smaller than the data region 35 if more than one reading and writing element were used for each magnetic shift register. For example, if two reading and writing elements were used for one shift register and were disposed equally along the length of the data region, then the reservoir would only need to be approximately half as long as the data region.
An electric current 45 is applied to the track 11 to move the magnetic moments within domains 25, 30 along the track 11, past the reading device 20 or the writing device 15. In a magnetic material with domain walls, a current passed across the domain walls moves the domain walls in the direction of the current flow. As the current passes through a domain, it becomes “spin polarized”. When this spin polarized current passes through into the next domain across the intervening domain wall, it develops a spin torque. This spin torque moves the domain wall. Domain wall velocities can be very high, i.e., on the order of 100 msec, so that the process of moving a particular domain to the required position for the purposes of reading this domain or for changing its magnetic state by means of the writing element can be very short. The domains are moved (or shifted) back and forth over the writing device 15 and reading device 20, in order to move the data region 35 in and out of the reservoir 40. Additional details regarding racetrack memory can be found in U.S. Pat. Nos. 6,834,005, 6,920,062, 7,551,469 and 8,638,601, for example.
The spin torque that is used to move the domain walls can be generated from several distinct physical phenomena that can be used to generate currents of spin polarized electrons. The most straightforward mechanism is derived from spin-dependent scattering in the interior of the magnetic materials that form the racetrack itself. The scattering rates for majority and minority spin polarized electrons can be very different depending on the detailed composition of the magnetic materials. Another mechanism that generates spin polarized currents is the Spin Hall Effect (SHE) in nominally non-magnetic layers in proximity to the magnetic layers that form the racetrack itself. The SHE is derived from spin orbit coupling in these layers that converts charge currents into pure spin currents that flow in a direction perpendicular to the charge current with a spin polarization direction that is both perpendicular to the current direction and to the spin current direction. These spin currents can give rise to highly efficient motion of domain walls, especially for racetracks that are formed from perpendicularly magnetized materials, via a chiral spin torque mechanism1-2.
A key principle underlying racetrack memory is the controlled creation3-5 and manipulation6-13 of domain walls in magnetic racetracks. This concept is also at the heart of several proposed logic14-15 and other proposed memory16 devices. Domain walls are also of interest as artificial traps of magnetic entities including magnetic nano-particles and atoms with magnetic moments such as in ultracold atom gases17. These rely on the use of magnetic fringing fields from either magnetic domains or magnetic domain walls to trap and transport the nanoscopic magnetic entities or atoms17. The racetracks can in general be formed from two types of magnetic materials, where the magnetization of the magnetic material can be either (i) primarily oriented within the plane of the nanowire or (ii) primarily oriented perpendicular to the plane of the nanowire. Materials of class (i) are typically composed of soft magnetic materials with small intrinsic magneto-crystalline anisotropies compared to the shape magnetic anisotropy derived from magnetostatic energies associated with the dimensions of the nanowire. Materials of class (ii) are typically composed of ultrathin magnetic layers in which their interfaces with non-magnetic layers give rise to interfacial magnetic anisotropies that can result in their magnetization preferring to be oriented perpendicular to these interfaces. For such materials the width of the domain walls decreases as the perpendicular magnetic anisotropy (PMA) is increased and can be as narrow as 0.5-10 nm, whereas in the case of magnetically soft materials (class (i)), the width of the domain walls scales with the width of the racetrack. Thus materials of class (ii) are preferred for the fabrication of dense racetrack memory devices (i.e., devices having high data densities).
The creation (sometimes also referred to as injection) of these domain walls in nanowires entails the controlled reversal of the magnetization in a localized region of the nanowire. Existing techniques for the domain wall injection have been based on the creation of local magnetic fields from nearby contact lines that are typically fabricated orthogonal to the nanowire. Other techniques for domain wall creation have been based on shaping and tuning the properties of the nanowire at a specific location in order to promote the likelihood of magnetization reversal from that section. This allows for the creation of domain walls through the use of a global magnetic field. However, these techniques are not applicable for denser racetrack memory devices, where the placement of many magnetic nanowires in close proximity is required, since individually addressing which magnetic nanowire a domain wall is injected into becomes difficult.
In either of these two cases (i) and (ii), however, the use of local magnetic fields to reverse the magnetization thus requires very large currents while also requiring the addition of several peripheral circuitries, which renders it not useful for dense racetrack memory devices.